A server admin is working on the issue.
Update 08:30: A server software update has gone terribly wrong. All customer data is fine and there are lots of backups, but it looks like we might have to build a new server from scratch. We're working on it.
Update 09:00: A new server is now being configured.
Update 11:00: New server configuration is about 90% done. We will begin moving sites to the new server shortly.